Why should you?
A simple example:
1. free app, only local app with, no community records, with ads and a low number of lives but with in app purchase option to buy more lives
2. a paid app, global app that you can see your records against every one else, no ads and double the amount of lives as the first app, but users asks also to be able to buy more lives if they finish what they get in the begging.
so, in conclusion, there are some cases when you need the flexibility of paid app that allow user to buy more things in the app even when they paid for the basic app.
Can it be done? I Couldn't find anything about it, only about freemium apps or free with in app purchases.
Yes, paid apps can include in-app purchases, including one-time purchases and subscriptions.